This invention relates in general to control systems for utilization with well pumps and in particular to control systems which vary the speed of the driving motor during each cycle of the well pump.
In the prior art it is known to utilize well pump control systems which remove power from the motive force for long periods of time to avoid operating the well pump when no oil is present in the underground pool. Such systems generally operate based upon the flow rate of oil from the well and are necessary to avoid a condition known as "fluid pound" which occurs when the pump plunger compresses expanded gases in the top of the chamber above the pump and the pump contacts the surface of the liquid in a sudden manner. This condition causes vibration and oscillation which may be destructive to the pump and production tubing.
As important as the avoidance of "fluid pound" is the avoidance of sudden vibration and oscillation which occurs as the reciprocating member in the well pump is suddenly lifted from the bottom of each stroke when fluid is present is very significant as is "pump off". The substantially incompressible nature of the fluids involved and the inertia which must be overcome at the beginning of each stroke of the pump create a sudden and substantial strain on the well pumping equipment. To compensate for the wear on well pump equipment and to increase the operability of electric motors utilized with such pumps, it is common to design electric motors which are to be utilized in well pumps with a high amount of "slip" capability. This slip is the amount of slip from synchronous motor speed that occurs as the motor attempts to overcome the weight of the fluid column on top of the pump at the beginning of each stroke. This slip generates high currents and great heat and it is quite expensive to design an electric motor to permit such slip. Additionally, the high current peaks generated during such slip operation and the fact that the motor is substantially unloaded during each downstroke causes a poor power factor and an attendant increase in operating costs.
Recently the aforementioned U.S. application Ser. No. 398,085, and now U.S. Pat. No. 4,476,418, by the inventor herein, proposed a system which removes electrical power from a standard drive motor for a selected period of time during each cycle to let the reciprocating member "coast" until its inertia has been overcome to a selected point. A tachometer is utilized to measure the speed of the motor while it is coasting and to couple electrical power back to the motor at a selected point during each cycle. While this novel control system does permit such a system to operate more efficiently without the necessity for high slip motors, the strain on the reciprocating member cannot be effectively controlled utilizing this system.